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From: Jonathan Guerin <jonathan@guerin.id.au>
Date: Tue, 14 Sep 2010 15:53:06 +1000
Message-ID: <AANLkTikvdkR=w-vGL=0LHnxQSibfoFBpuhLwjJZWVCuM@mail.gmail.com>
Subject: Re: [support] ath5k contention windows
To: linux-wireless <linux-wireless@vger.kernel.org>,
	ath5k-devel <ath5k-devel@lists.ath5k.org>
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Hi all,

I have some behaviour I'm observing with some Atheros cards we use
that doesn't seem to match what the initvals of ath5k are set up to.
These are the cards I used:
http://www.mini-box.com/s.nl/it.A/id.387/.f

I have run a saturated iPerf flow on a conducted testbed with both
stations being inside RF-shielding boxes. They are set to 802.11a
mode, on channel 1. I then parse the trace, looking for ACK-DATA
pairs, and calculating the time difference between them. From this, I
remove the TX_TIME of the DATA frame, as well as a DIFS:

ACK_TIMESTAMP + DIFS + CONTENTION_TIME + DATA_TX_TIME = DATA_TIMESTAMP

which will leave me with the CONTENTION_TIME. Dividing this time by a
SLOT_TIME will give me the slot which was chosen by the hardware.


According to the driver, in ath5k.h:

#define AR5K_TUNE_CWMIN                15

CWMIN is initialised to 15.

The actual distribution of contention slots I'm observing resembles this:

Slot Number,Count
0,1315
1,1302
2,1249
3,1291
4,1347
5,1219
6,1249
7,0
8,0
9,0
<truncated>

as well as 1360 frames which came in with a negative CONTENTION_TIME.

Ignoring the fact that some frames are coming up with a negative
CONTENTION_TIME (which potentially points to another problem), what is
being observed here is that CW_MIN appears to start at 7, rather than
the 15 which it should be.

I'm just wondering if anyone would have any idea why this is occurring?

Thanks,

--
Jonathan Guerin

PS sorry for the double-post, the system complained as my message was
originally in HTML